Protocols used to transmit digital data typically may use some form of multi-carrier modulation such as orthogonal frequency division multiplexing (OFDM) or scalable adaptive modulation (SAM). As is known by those skilled in the art, multicarrier modulation schemes often use some sort of pilot symbols that are embedded within the transmitted waveform. The pilot symbols are typically used for channel quality estimation such that various parameters of a transmitted signal can be controlled depending on the received signal conditions on that particular channel. In a wireless communications environment, channel conditions vary over both time and frequency. It is important for any wireless communications system to be able to overcome varying channel conditions and estimate received symbols accurately. Typically, multicarrier modulation schemes employ methods of pilot interpolation to determine the effect of the channel gain on each data symbol. In the multicarrier transmitter, pilot symbols are inserted into the data stream at regular intervals in all the subchannels. The receiver uses the received pilot symbols to estimate changing channel conditions. If PT is a transmitted pilot symbol and PR is the value of the corresponding received pilot symbol, then the relationship between PT and PR can be described by the equation PR=(g)(PT) where g represents the complex gain. Knowing the values of PR and PT, it is easy to obtain g for each transmitted pilot symbol.g=PR/PT
Pilot interpolation is the name given to an algorithm that is used to determine the effect of the channel on its data symbols based upon a series of received pilot symbols. In other words, the pilot interpolation is to compute the value of complex gain for each received pilot symbol and interpolate the value of the complex gain and obtain gain for the data symbols. Once the gain for each data symbol is obtained, the data symbols are scaled by the gain and passed on to a hard/soft decision decoder. In most multicarrier type modulations, the complex gain (g) includes both the effect of the channel and the phase offset effects from the mixers. As known by those skilled in the art, the pilot interpolation algorithm used in SAM and other multicarrier modulation schemes uses a weighted combination of pilots in the same and adjacent sub-channels to estimate the channel effect. The sync symbols (which occupy the first two bauds across all sub-channels) are also used in the pilot interpolation process.
As the pilot symbols are received at a receiver an associated algorithm is used to produce a set of interpolation coefficients. These coefficients are in turn used to determine how to process and/or interpret the incoming data for a given set of parameters. This allows the incoming data to be processed with the least amount of errors for a set of given conditions. As known to those skilled in the art, changes due to channel fading are typically due to variations in time and in frequency. Typically, only one set of coefficients is used for all fading channel conditions including both fast and slow channel fading as well as any changes in frequency such as flat fading and selective fading. The use of only one set of channel coefficients can impose problems when the receiver is moving at various speeds that cause these varying signal conditions. For example, one coefficient set often does not work well when the receiver moves from a stationary position to various speeds such as 5 kph, 70 kph and/or 200 kph. This type of scenario might be one that would be encountered in a helicopter or other vehicle having a wide range of speeds. The problem in making a channel estimation calculation centers around determining a method to obtain the channel information such as vehicle speed and fading selectivity. The global positioning system (GPS) can be a solution where a vehicle's speed can selectively be calculated from its movement and location. However, methods using GPS are neither easy to implement nor are they cost effective.
Thus, the need exists for a method to measure given signal conditions and the selection of an appropriate set of pilot coefficients based upon channel estimation without using GPS.